A wavelength selective switch (WSS) is a free space optical system that has relatively long optical path lengths. Within that optical system the beam is redirected by reflection, refraction and diffraction. Of these, the beam propagation direction upon diffraction and refraction is sensitive to the ambient index of refraction of the optical system, e.g. the index of refraction of the gaseous medium between the optical elements. Changes in the index of refraction of that medium result in a change in the beam propagation direction and a degradation in the WSS optical performance. There are two ways that changes in the index of refraction of the medium can come about:
1) Changes in the composition of the medium can change the index of refraction. Composition changes can be a result of outgassing of elements within the package or as a result of gas exchange through a leak in the package. Typically, the requirements for package hermeticity is 5×10−8 atm cc/s Helium leak rate.
2) Changes in the density of the medium can also change the index of refraction. Changes in density can result from changes in the package volume in response to external pressure fluctuations. Typically, volume change are required to be less than 1%, so as not to impair the optical performance excessively. Note that structural rigidity can also be important in cases where the optics are rigidly coupled to the external package, as is the case in some compact WSS's, since package deformation under external forces can couple directly to the optics.
Varying external temperature may also result in changes in the package volume, and the result is a temperature dependent index of refraction for the free space medium surrounding the optics, because a temperature change in the volume changes the density, which changes the index of refraction. In this case, however, through selection of optical materials and their dn/dT, the variation of index of refraction with temperature, the dnmedium/dT may be compensated for with the dnglass/dT.
Hermetic packages for optical devices represent a significant cost element, which arises from the fact that current methods, utilizing a single structural-hermetic package, necessitate design compromises to simultaneously meet the dual requirements of hermeticity and structural rigidity. These design compromises tend to increase the package cost. Examples include:
1) The use of specialized alloys, e.g. Kovar™, as a package material for their thermal expansion (CTE) properties with high temperature co-fired ceramic (HTCC) and glass to metal seal (GTMS) electrical feedthroughs.
2) Use of specialized hermetic suppliers, to produce the entire package. Unfortunately, because all the elements are integrated into a single package, the supplier delivers, not only the hermetic electrical feedthrough in which they specialize, but also the less demanding elements of the package.
3) package-wide plating requirements and tight tolerances driven by hermetic processes.
By separating or decoupling the structural and hermetic design elements, each can be delivered at substantially lower cost.
Glass to Metal Seal (GTMS) and High Temperature Co-fired Ceramic (HTCC) are typically used to achieve the electrical feedthrough function in hermetic packages. Each of these sealing methods requires that the CTE of the package material is strictly matched to the CTE of the feedthrough material. Kovar is well CTE matched to the HTCC material, and Kovar may also be used to form a compressive seal around a glass feedthrough. Unfortunately, Kovar is quite an expensive material, and the fact that it is difficult to machine only adds to the cost of Kovar packaging.
The present invention seeks to reduce the amount, i.e. package area, of Kovar that is required to interface to the HTCC or GTMS, and to replace the balance of the hermetic enclosure with a less expensive material.
In some WSS products a Kovar/GTMS cap, which gets sealed to an aluminum package body, is used, because aluminum is much cheaper than Kovar. However, the available sealing method for hermetically sealing Kovar to aluminum, e.g. solder, drives stringent requirements on plating, i.e. to enable the solder to wet the surface and make a good seal, and on the machining of the mating surfaces of the seal, i.e. to ensure a complete seal without solder voids while at the same time avoiding solder spills. In some prior art systems, a machined tongue and groove arrangement, which is gold plated, is used to achieve a good seal. Unfortunately, the cost of machining and of the gold drives the total cost up.
An object of the present invention is to overcome the shortcomings of the prior art by providing packaging for an optical device, which decouples structural and hermetic requirements.